1. Field of the Invention
This invention relates to silicone rubber compositions convertible to elastomers having highly improved mechanical strengths, especially tear strength.
2. Description of the Prior Art
Silicone rubber elastomers are generally recognized for their outstanding heat-resistance and electric characteristics, but are considered disadvantageous with respect to their inferior mechanical strengths, especially tear strength, as compared to other synthetic or natural rubbers. This results in restriction in the applications of these materials.
Various methods have been proposed for imparting greater mechanical strengths to silicone rubbers. The proposed methods include (i) a method comprising curing a blend of diorganopolysiloxane gums with minor and major amounts of vinyl groups attached to silicone atoms in the presence of an organic peroxide, so that the resulting cured elastomers may have a localized distribution of crosslinking points (see U.S. Pat. No. 3,660,345); (ii) a method comprising incorporating in the rubber stocks, a reinforcing silica filler treated with a silane or siloxane having, particularly, alkoxy or silanol groups, so that the physical properties of the resulting elastomers can be improved (see U.S. Pat. Nos. 3,024,126 and 3,122,520); (iii) a method comprising incorporating fibrous asbestos so that the reinforcing effect may be enhanced without impairing the heat resistance of the resulting elastomers (see Japanese Public Disclosure No. 48-80656); and (iv) a method comprising formulation with the block copolymers of organopolysiloxane, so that the resulting elastomers may have a localized distribution of crosslinking points (see U.S. Pat. Nos. 3,665,052, 3,668,273, 3,701,815, 3,781,378 and 3,819,744).
The above methods can, indeed, impart high tear strength to the silicone rubbers, but are defective in that they tend to lower the modulus of the rubbers due to decreases in the crosslinking density and also the occurrence of relatively long segments between the crosslinking points. For example, the silicone rubbers obtained by the known methods have been found to give poor results in the cut-through test, penetration test and crushing test when employed as an insulating material for electric wire coverage.